In typical power and free conveyors, a load carrier is mounted on a load trolley or trolleys which run on a load or free track. The load carrier is moved by a continuously running power or drive chain by engagement between a drive or pusher dog on the drive chain and a retractable trolley dog on the load carrier or trolley. The drive chain is supported by power trolleys running along a power track. In overhead power and free conveyors, a load supported by the load carrier is suspended below the tracks carrying the load trolleys and the drive chain. The pusher dog extends downward to engage an upwardly extending trolley dog. The trolley dog may be retracted from the pusher dog to allow the load to coast on a downhill section of the load track, to halt the load for operations thereon, or the like. On many power and free conveyors, the load trolleys incorporate accumulation mechanisms which cause drive disengagement of carriers approaching behind a halted carrier to prevent collisions between the carriers.
Inverted power and free conveyors are similar to overhead power and free conveyors except that, as their name suggests, they are turned upside down. On inverted power and free conveyors, a power track supporting the power trolleys carrying the drive chain is at the lowest level. Above the power track is the load or free track supporting the load trolleys, with the load carrier above the load track. The pusher dogs of inverted power and free conveyors extend upward to engage downwardly extending trolley dogs which may be retracted to disengage drive from the load for the same reasons as for overhead track conveyors. Both overhead and inverted power and free conveyors find application in factories, such as on automotive assembly lines to carry automotive bodies as manufacturing operations are performed, in large appliance manufacturing plants, and the like.
Carriers, particularly for inverted power and free conveyors, are usually provided with two spaced apart load trolleys to stabilize the load from pivoting about a lateral axis. The load carrier structure extends between the two load trolleys and has the load clamped, or otherwise temporarily fastened, thereto. Load carriers for overhead power and free conveyors may also be provided with two spaced apart load trolleys to control swinging or rocking about a lateral axis. One of the problems of operating a power and free conveyor is that the engagement of a drive dog on the drive chain with the trolley dog is often abrupt. That is, a stationary load carrier is jerked into motion with minimal slowdown of the drive chain. Braking of the load carriers is also often abrupt. Shock generated by this abrupt engagement of the drive dog with the load carrier or braking of the carrier can be transferred to the load carried with the possibility of damage to the load or disengagement of the load from the load carrier.
In order to reduce the transmission of shock to carried loads from the startup and braking of the load carriers and reactions in the drive chain to engagement of a pusher dog to a trolley dog on a carrier, which may be transmitted to other loads carried on the conveyor, various shock absorbing arrangements have been devised. The simplest types of shock absorbers involve resilient padding, such as rubber bushings or the like which are positioned in connecting parts between the load trolleys and the carrier frame. Such padding is only marginally effective in reducing the transmission of shocks to the loads. Other types of shock absorbers involve the use of springs between sliding connections of the trolleys and carrier frame. The principal problem with springs alone is that once compressed, for example, they recover resiliently and often cause periodic motion or longitudinal bouncing of the load in the direction of travel. Such bouncing of the load strains the load, possibly damaging it, and is often almost as undesirable as an abrupt change in motion.
The most effective types of shock absorbing devices for conveyors of this type are those which damp the shock without converting it to bouncing motion or vibrations. Such devices operate in a manner similar to automotive suspension type shock absorbers and may be similar in construction. Shock absorbers of this type are typically hydraulic or pneumatic cylinders with sliding pistons therein which retard relative movement of the piston and cylinder by forcing the contained fluid through an orifice in the piston. Shock loads applied to one of the members is dissipated in viscous resistance to the flow of the fluid through the orifice and reduced in intensity and abruptness prior to application as motion to the other member of the device. Another known type of shock absorbing device, which operates in a manner analogous to fluid shock absorbers, involves a cylinder filled with a particulate material, such as metal shot or ball bearings, and has a type of piston moving through the material. In this type of cylinder, the shock is dissipated as the piston moves through the material by friction among the particles of the material and, to some degree, inertia of the particles.
Known conveyor arrangements employing true shock damping devices typically employ two trolleys to support the load carrier and load and a third drive trolley connected to the leading support trolley by the shock absorber. Such an arrangement increases the length of the carrier assembly required for each load, resulting in fewer loads capable of being positioned on a given length of conveyor or accumulation zone. Additionally, economic resources are wasted in the third trolley which does not actually support a load.